Where are the best places to go in London on a weekend for free?

Just walk. One of the joys of London is that any walk you take is guaranteed to turn up interesting architecture and surprises. I once spent an afternoon walking from Paddington to Tate Modern and saw loads of fascinating places that I had no idea were there. Use Google maps and look out for interesting looking squares, parks and back streets. These days I also tend to pop open Foursquare’s “Explore” tab every now and then to see if I’m walking near something particularly interesting.